Also, intersecting with https://chromium-review.googlesource.com/c/chromium/src/+/873351, we might only want to monitor device changes if a renderer is listening for updates (or has recently enumerated devices), and drop the cache otherwise. May be worth checking how the video capture service handles this.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d4af62ded717ec9fe2bb54d9d05439d4dfeff0b5

commit d4af62ded717ec9fe2bb54d9d05439d4dfeff0b5
Author: Guido Urdaneta <guidou@chromium.org>
Date: Thu May 10 17:09:57 2018

Add device monitoring to the audio service.

Bug:  823640 
Change-Id: Iede7cc3d9a7b7f39b89810914dcda1cb61379cdc
Reviewed-on: https://chromium-review.googlesource.com/1041865
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Olga Sharonova <olka@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557563}
[modify] https://crrev.com/d4af62ded717ec9fe2bb54d9d05439d4dfeff0b5/content/browser/renderer_host/media/media_devices_manager.cc
[modify] https://crrev.com/d4af62ded717ec9fe2bb54d9d05439d4dfeff0b5/content/browser/renderer_host/media/media_devices_manager.h
[modify] https://crrev.com/d4af62ded717ec9fe2bb54d9d05439d4dfeff0b5/content/public/app/mojo/content_browser_manifest.json
[modify] https://crrev.com/d4af62ded717ec9fe2bb54d9d05439d4dfeff0b5/services/audio/BUILD.gn
[add] https://crrev.com/d4af62ded717ec9fe2bb54d9d05439d4dfeff0b5/services/audio/device_notifier.cc
[add] https://crrev.com/d4af62ded717ec9fe2bb54d9d05439d4dfeff0b5/services/audio/device_notifier.h
[add] https://crrev.com/d4af62ded717ec9fe2bb54d9d05439d4dfeff0b5/services/audio/device_notifier_unittest.cc
[modify] https://crrev.com/d4af62ded717ec9fe2bb54d9d05439d4dfeff0b5/services/audio/manifest.json
[modify] https://crrev.com/d4af62ded717ec9fe2bb54d9d05439d4dfeff0b5/services/audio/public/mojom/BUILD.gn
[add] https://crrev.com/d4af62ded717ec9fe2bb54d9d05439d4dfeff0b5/services/audio/public/mojom/device_notifications.mojom
[modify] https://crrev.com/d4af62ded717ec9fe2bb54d9d05439d4dfeff0b5/services/audio/service.cc
[modify] https://crrev.com/d4af62ded717ec9fe2bb54d9d05439d4dfeff0b5/services/audio/service.h
[modify] https://crrev.com/d4af62ded717ec9fe2bb54d9d05439d4dfeff0b5/services/audio/service_factory.cc
[modify] https://crrev.com/d4af62ded717ec9fe2bb54d9d05439d4dfeff0b5/services/audio/test/in_process_service_test.cc
[modify] https://crrev.com/d4af62ded717ec9fe2bb54d9d05439d4dfeff0b5/services/audio/test/service_lifetime_connector_test.cc

[bulk-edit: disregard if N/A] Can the owner please set milestone to this bug if applicable?